Shear-sensitive plugging fluid for plugging and a method for plugging a subterranean formation zone

ABSTRACT

A plugging fluid for plugging a subterranean formation zone surrounding a drill hole comprises an emulsion comprising a dispersed aqueous phase containing an aqueous base and a continuous hydrophobic phase containing a polymer consisting of a grafted cellulose ether derivative, a surfactant and a crosslinking activator of the polymer. The polymer is preferably 2-hydroxyethyl cellulose grafted with vinyl phosphonic acid. Upon shearing, preferably through the drilling bit, the emulsion inverts so that the rupture of the emulsion droplets releases the crosslinking activator into the water phase thus forming a gel structure.

REFERENCE TO RELATED PROVISIONAL APPLICATION

This application claims the benefit of the U.S. Provisional ApplicationSer. No. 60/309,538 filed Aug. 2, 2001 and U.S. Provisional ApplicationSer. No. 60/334,444 filed Nov. 29, 2001.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a reactive plugging fluid designed togel rapidly when subjected to high shear stress. The invention alsorelates to a method for plugging a subterranean formation zone,especially for curing massive mud losses when drilling a well.

BACKGROUND OF THE INVENTION

Lost circulation while drilling is a major problem. The well costdramatically increases due to the lost time from delayed well productionand also due to associated drilled problems such as pipe sticking andsafety issues. The most common technique to combat lost circulation isto add into the drilling fluid a lost circulation material (LCM).Granular flakes and fibrous particles, essentially based on cellulosicmaterials, are used for sealing off fractures, vugs and porous zones.Minerals such as mica are also commonly used. If even highconcentrations of lost circulation materials fail to restore thedrilling fluid circulation, a cement plug is placed. The cement plugconsolidates the voids but also fills the open wellbore and needs to bedrilled before continuing the well drilling. Quite often, the proceduremust be repeated several times before achieving a correct seal.

Other techniques involve the use of reactive fluids. Two reactive fluidsare either mixed near the formation where lost occurs with a first fluidpumped through the drill-string and a second fluid displaced down theannulus. At the interface of the two fluids, the turbulent flow allowsthe rapid formation of a rubbery solid mass commonly known as a gunk.Another practice uses crosslinked polymer gels whose reaction isinitiated on surface. In both case, the technology is highly risky sinceslight changes in the composition, temperature or fluid contaminationmay lead to premature gellation in and around the bottom hole assembly,leading to major operation failure.

It is also known to use as plugging fluids so-called rheotropic liquidsthat thicken when subjected to high stress. U.S. Pat. No. 4,663,366discloses such a polycarboxylic acid containing water-in-oil emulsionwhere the oil phase contains hydratable water-swelling hydrophilic claysuch as bentonite and the aqueous phase contains a dissolvedpolyacrylamide and a polycarboxylic acid. The setting of this pluggingfluid takes place as a result of a swelling of the bentonite whenbentonite contacts water. Each dispersed droplet of the aqueous phase iscoated with a polymeric material so that the contact only occurs whenthe emulsion is subjected to high shear forces that break this coating.

Another rheotropic plugging fluid is known from U.S. Pat. No. 5,919,739(Sunde et al.). Like the emulsion of the U.S. Pat. No. 4,663,366, thefluid is based upon a ‘loose’ invert emulsion. The continuous phaseprovides an encapsulation medium for a crosslinker and the internalphase consists of a high concentration of a polymer while theinterfacial tension between the two phases is maintained by aconcentration of a lipophilic surfactant.

A preferred plugging fluid of the Sunde et al. patent applicationconsists of about 25% by volume of a continuous phase containing anhydrophobic liquid selected from mineral oils, vegetable oils, estersand ethers, an emulsifier on a triglyceride basis, bentonite and calciumhydroxide and of about 75% by volume of a dispersed aqueous phasecontaining water, xanthane and optionally, a weighting material such asbarite. When this type of fluid experiences a significant pressure drop,an inversion of the emulsion occurs and the crosslinker is released intothe aqueous phase resulting in the formation of a gel.

This latter type of plugging fluid can be stored for several weekswithout reacting and pumped with a centrifugal pump for several hours.Gellation is fast and triggered only by subjecting the plugging fluid tohigh shear forces, for instance when forced through the drill bit.However, the use of this type of plugging fluid is limited by lack ofrobustness and shrinkage over time. Moreover, above a temperaturethreshold of about 90° C., the gel becomes less rigid and turns into aviscous fluid due to the breaking of the crosslinked bonds.

It would therefore be desirable to provide a new plugging fluid suitableto effectively seal off the problem zone and stable across a widetemperature range. There is also a need in well control for betterprocedures, including placement strategies to help in making jobssuccessful.

SUMMARY OF THE INVENTION

Thus, the invention provides a plugging fluid for plugging asubterranean formation zone surrounding a drill hole consisting of anemulsion comprising a dispersed aqueous phase containing an aqueousbase, and a continuous hydrophobic phase containing ahydroxyethylcellulose derivative graft polymer, a surfactant and acrosslinking activator of the hydroxyethylcellulose polymer.

The emulsion is believed to be invert (water-in-oil) though it mightactually be direct (oil-in-water) with further water droplets within thelarge oil droplet, i.e. an invert emulsion in a direct emulsion.

The grafter polymers useful to carry out the invention are celluloseether derivative with vinyl phosphonic acid grafts.

The cellulose ether derivative is preferably a hydroxyalkyl cellulosewhere the alkyl is selected from the group of ethyl and propyl. Thepreferred hydroxyalkyl cellulose is 2-hydroxyethylcellulose. A processfor preparing cellulose ethers having at least one phosphorouscontaining substitute is known from U.S. Pat. No. 4,413,121, which ishereby referred to and incorporated by reference.

The principle of the setting of the plugging fluid of the presentinvention is essentially the same as for the plugging fluid of U.S. Pat.No. 4,663,366 discussed above. It is the crosslinking of the graftedhydroxyalkyl cellulose that causes the gel formation. Crosslinkablecellulose derivatives are known as state-of-the-art polymericviscosifiers used in the oil industry in particular for controllingfluid loss in subterranean formations. Reference is made for instance toU.S. Pat. Nos. 5,439,057 and 5,680,900. A crosslink bond is createdbetween a metal ion and the pendant groups along the polymer chain ofthe polysaccharides. Upon exposing the plugging fluid to a pressure dropgreater than 2 MPa over a small dimension, it is believed that theemulsion inverts or flips from its invert state into a more stabledirect state. The rupture of the emulsion droplets releases thecrosslinker activator into the water phase thus providing metallicdivalent ion to crosslink with the cellulose ether derivatives andforming the gel structure.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The preferred polymer utilized in this invention is a derivatizedhydroxyethylcellulose, more specifically, 2-hydroxyethyl cellulose,vinyl phosphonic acid graft polymer. The ratio of 2-hydroxyethylcellulose to vinyl phosphonic acid monomers in the graft polymertypically ranges from about 5% to about 20% and a preferred ratio isfrom about 10% to about 12%. Range of concentration is from 0.1 to 2 wt%, with 2 wt % generally preferred. Increased polymer concentrationsresult in more rigid gels.

The crosslinker agents or crosslinker activator admixtures are selectedfrom the groups consisting of iron III compounds, boron (such as boricacid, borate salts), titanium IV, zirconium IV, aluminum III, antimonyV, compounds containing divalent calcium and magnesium ions (for examplemagnesium oxide and calcium oxide), amines (mono, di or trialkanol;cocoamines, pentamines, alkyldiethanol amines), acetates (such as sodiumacetate), sodium hydroxide, potassium hydroxide, and buffers, such assodium carbonate, potassium carbonate, and sodium acetate; and/or otheradditives which generate hydroxyl ions in aqueous solutions, such asammonia, ammonia compounds and ammonia generating compounds, forexample, urea; chelating agents such as sodium lactate, salts ofhydroxyethylaminocarboxylic acid—such ashydroxyethylethylene-diaminetriacetic acid (EDTA), sodium gluconate,sorbitol, and glycerol.

The preferred crosslinking/pH control agents are magnesium oxide, sodiumcarbonate, EDTA, and sodium tetraborate. The concentration range formagnesium oxide is from 0.01 to 3 wt %, preferably from about 0.5 toabout 1.5 wt %. The concentration range for sodium carbonate is from0.01 to 3 wt %, preferably from about 0.1 to 1 wt %. The concentrationrange for EDTA is from 0.01 to 3 wt %, preferably from about 0.1 to 1 wt%; and the concentration range for the sodium tetraborate is from about0.01 to 3 wt %, preferably from about 0.1 to 1 wt %.

Any clean liquid hydrocarbon can be used for the oil phase. The oil mayadvantageously be selected from any base oil suitable for drillingfluids such as mineral oils, vegetable oils, esters and ether oils,diesel, alpha-olefins, polyolefines, n-alkanes, and mixtures thereof.Selected oils must be of compatible with the used drilling fluids andthe environmental regulations that for instance prohibit use of aromaticcontaining oils on offshore rigs. The concentration range for the oilphase is from about 5 to about 70 wt %, and preferably from about 10 toabout 25%.

The used water is preferably fresh tap water. Non-contaminated drillwater such as seawater or light brine diluted with about 50% fresh watercan also be used. The concentration range is from about 30 to about 95wt %, more preferably from about 70 to about 90 wt %.

The crosslink bond created between the metal ion and the hydroxyl groupsconcurs if the pH is ranged between 11 and 13.

Conventional commercially available emulsifiers can be used, selected onthe basis of their compatibility with alkali environment and theintended temperature of use. Lipophilic surfactants, used to preparewater-in-oil emulsion drilling fluids comprising a high water content(greater than 50% by volume) and known to those skilled in the art canprovide the required emulsion strength. Preferred emulsifiers are basedon combination of fatty acids and polyamides or on triglyceride. Thesetting time of the plugging fluid depends on the amount of emulsifier:the addition of a small amount of emulsifier will result in a very shortsetting time and a highly unstable emulsion. Conversely, largeconcentration of emulsion will result in a too stable fluid, a very longsetting time and higher shear pressures required for gelling. Thepreferred surfactants are polyamide derivatives, with concentrationsranging from about 0.01 to about 5 wt %, and most preferably from about0.1 to about 3 wt %, to control the shear rate required to invert theemulsion.

The plugging fluid according to the present invention may optionallycomprise additional additives such as a setting accelerator, lostcirculation material and extenders.

The plugging system according to the present invention may be used to anextended range of temperatures, ranging from about 40° F. to about 325°F. (about 4° C. to about 163° C.).

Though it is not preferred, the plugging fluid may also compriseconventional lost circulation material such as nut plug, fibers, calciumcarbonate, mica etc. If the plugging fluid is subjected to pass throughthe drill bit, then the lost circulation materials must be of fine ormedium grade depending on the size of the jets at the bit.

Extenders such as clays are commonly used in wellbore fluid to improvethe suspension of solids, to keep particulate solids, such as bridgingagents, from separating. Bentonite is preferred due to its ability toabsorb large amounts of water, thus preventing dilution of the gel bywater influx. Bentonite further increases the gel strength of the slurryand improves the suspension of solids when lost circulation material isfurther included.

The plugging fluid of the invention can also be used in conjunction withweighting agents. The weighting agents are selected from the groupconsisting of barite, ilmenite, hematite, manganese and calciumcarbonate. With the solid weighting additives added to the oil phase,the density may be typically adjusted between 0.98 g/cm³ (no weightingagent) and 1.68 g/cm³. Where the solid weighting additives are added tothe aqueous phase, higher density may be achieved, up to about 2 g/cm³.

A preferred plugging fluid according to the present invention comprises15.65 wt % of diesel oil, 0.18 w % of a polyamide derivative(emulsifier), 1.96 w % of a derivatized hydroxyethylcellulose(2-hydroxyethylcellulose 89-90 w %, vinyl phosphonic acid graft polymer10-11 wt %, molecular weight 1,300,000), 78.27 w % of fresh water, andas crosslinking agent/activator, 1.27 w % of magnesium oxide, 0.51 w %of sodium carbonate, 0.67 w % of sodium tetraborate pentahydrate and0.51 w % of the tri-potassium salt of EDTA.

According to a preferred variant of the present invention, the methodfor preparing the new plugging fluid comprises the steps of dissolvingthe emulsifier into the oil, under gentle agitation (for instance about400 RPM), for about 2 minutes, adding all other liquid or solidadditives, including the polymer, to the base oil, under the same gentleagitation for about the same period of time to prepare a pre-mix thatcomprises all the constituents of the emulsion but water and addingwater for instance with a static mixer immediately before pumping itdownhole so that the polymer does not migrate into the aqueous phase andconsequently, is not hydrated before the breakage of the emulsion. It isworth noting that a substantial migration and hydration of the polymeris obtained in a period of several minutes, for instance between about15 and 30 minutes. According to this preferred embodiment of theinvention, the emulsion is pumped less than 1 minute, and preferablyabout 30 seconds after the mixing so that no significant hydration canoccur in such a small period of time.

This pre-mix can be stored and leave on location until needed providedunplanned addition of water is prevented to avoid a dramatic increase ofthe viscosity.

One aspect of the present invention is a method for placing a plug in awell bore to treat lost circulation. The recommended practice afterencountering a lost circulation zone is to treat the zone as soon aspossible. The depth of the lowest lost circulation zone can be found bylogging (for instance with imaging tools) or by plotting depth versusloss rate. Accurate location is a key to make sure that the pluggingpill will be placed below the area of lost circulation. The pill ispreferably pumped through the drill bit nozzles using pills having avolume of about 5-15 m³. After the pumping of a first pill, the well isallowed to equilibrate and attempts should be made to attemptcirculation. If full circulation is not obtained, then two or threepills may be needed to effectively seal the zone.

These and other features of the invention will become appreciated andunderstood by those skilled in the art from the detailed description ofthe following examples.

Laboratory Evaluation & Example

A fluid pill was mixed according to the preferred formulation, thusconsisting of 80 g of diesel oil, 0.94 g of a polyamide derivative(emulsifier), 10 g of a derivatized hydroxyethylcellulose(2-hydroxyethylcellulose 89-90 w %, vinyl phosphonic acid graft polymer10-11 wt %, molecular weight 1,300,000), 400 g of fresh water, and ascrosslinking agent/activator, 6.5 g of magnesium oxide, 2.6 g of sodiumcarbonate, 3.4 g of sodium tetraborate pentahydrate, 5 g fine silica and2.6 g of EDTA (tri-potassium salt).

The oil is placed in a vessel and agitated. To the agitated oil phasethe surfactant is added slowly until the surfactant is dissolved in theoil, under agitation of at 400 RPM for 2 minutes. Solid additives(Magnesium oxide, sodium carbonate, polymer, sodium tetraboratepentahydrate, fine silica and Ethylenediamine tetra-acetic acid) arenext added (in any order) to the base oil under agitation 400 RPM for afurther 2 minutes. The emulsion is formed by slowly adding water to theagitated oil phase and the while increasing the blender speed to amaximum of 600 rpm in order to obtain the desired emulsion in less than30 seconds.

Procedure for Inverting the Emulsion with Shear

The emulsion is inverted on exposure to shear. The shear can besimulated in the laboratory in a number of ways. As an example, threedifferent methods are provided below: using a shearing unit, using amodified API fluid loss cell, and using a blade-type mixer (Waringcommercial blender).

1) Using a Shearing Unit—similar to that described in U.S. Pat. No.5,717,131

The emulsion is poured in the reservoir, the pressure inlet is adjustedto 7 bars (100 psi) and the shearing nozzle is set at 35 bars (500 psi)by initially testing with a low viscous fluid. The flipped emulsion iscollected at the outlet in a plastic beaker or small plastic cubes, or arubber hose for subsequent extrusion tests.

2) Using a modified API fluid loss cell

In a high temperature high-pressure fluid loss cell used normally to runfluid loss tests for cement slurries. A piston equipped with two O-ringsto have a good contact is added into the cell. The piston is pushed tothe bottom of the inverted fluid loss cell. The emulsion is poured intothe fluid loss cell and a spacer without the filter screen is added toprevent leakage and the cap is fixed. This cap is equipped with a valvewith a ¼ inch end. The fluid loss cell is reversed and placed on itsstand and connection for pressure is mounted. A pressure of 35 bars (500psi) is applied. The top valve is opened allowing pushing the emulsionvia the piston. The bottom valve is shortly opened and closed to preventprojections and the flipped emulsion is collected in a plastic beaker orsmall plastic cubes.

3) Using a Blade-Type Mixer

After the emulsion is prepared, it is mixed for 20 seconds at 7,000 RPMin a blade-type mixer. The flipped emulsion is poured in a plasticbeaker or small plastic cubes.

Extrusion Test Method

Emulsion is prepared as mentioned above and flipped with the shearingunit. At the outlet of the shearing unit a rubber hose is connected. Theinverted (“flipped”) passes through the hose, filling it completely. Assoon as the flipped emulsion escapes the hose, the hose is disconnectedfrom the shearing unit and connected to a pressure line. Pressure isadjusted with a regulator allowing nitrogen gas to push the gel. Thepressure needed to extrude the gel is read on the digital gauge andrecorded. This extrusion test can be performed on gel in the rubber hoseimmediately after flipping or it can be allowed to age for a period ofhours prior to extruding.

It is also possible to connect the rubber hose to the modified API fluidloss cell in order to flip the emulsion in the rubber hose. Extrusiontests can then be performed on the gel.

Data for Preferred Formulation Performed at Ambient Temperature

Hydration Pressure time of required to Pressure Pressure the preparedextrude required to required to Emulsion emulsion the gel extrude thegel extrude the gel sheared prior to immediately after aging in afteraging in with shearing after the rubber hose the rubber hose the . . .(hour) shearing for 1 hour for 2 hours shearing 1 16 psi 84 psi unit[230400 lb/ [1209600 lb/ 100 ft²] 100 ft²] fluid loss 1 12 psi 60 psi 70psi cell [172800 lb/ [864000 lb/ [1008000 lb/ 100 ft²] 100 ft²] 100 ft²]fluid loss 17 12 psi 80 psi cell [172800 lb/ [1152000 lb/ 100 ft²] 100ft²]

The gels were sheared at room temperature and placed at temperature upto 325° F. The gel was then cooled and visually inspected. In anothertest, the emulsion was taken to 300° F. prior to shearing andsubsequently sheared. In both cases, a strong gel was formed even if thegel formed at 300° F. was actually stronger.

Yard Test

A yard test was conducted to evaluate the formation of an emulsionthrough a static mixer, as it might be suitable for offshoreapplications. The static mixer improved the contact between the oilphase and mixing water in order to create a homogenous emulsion. Arestriction (choke) was used to provide the pressure drop required toflip the emulsion and form a gel.

An un-weighted emulsion was prepared, consisting of 320 liters of dieseloil (base oil), 19.2 liters of a polyamide derivative (emulsifier),24.09 kg of a crosslinking activator comprising magnesium oxide,dihydrogenated tallow dimethyl ammonium bentonite and ethoxylated octylphenol, 4.82 kg of soda ash as a buffer, 48.23 kg of a viscosifier, aderivatized hydroxyethylcellulose (2-hydroxyethylcellulose 89-90 w %,vinyl phosphonic acid graft polymer 10-11 wt %, molecular weight1,300,000), 12.53 kg of sodium tetraborate decahydrate, 9.64 kg of tetrasodium ethylenediaminetetraacetate (tetrasodium EDTA), 24.09 kg ofcrystalline silica powder and 1927 liters of water.

A weighted emulsion was also prepared, consisting of 357.7 liters ofbase oil, 14.31 liters of the emulsifier, 17.89 kg of the crosslinkingactivator, 3.58 kg of the buffer, 35.78 kg of the viscosifier, 9.30 kgof sodium tetraborate decahydrate, 7.16 kg of tetrasodium EDTA), 17.89kg of crystalline silica powder and 1430 liters of water.

Before mixing the oil phase, the choke was calibrated with water and allthe lines were pressure-tested. Two barrels (317.97 liters) of oil wereadded, along with the emulsifier, to the tub, circulated and displacedto the discharge tank to remove any water in the lines.

The emulsifier was first added to the oil phase in the tub. The mixturewas allowed to mix for two minutes. The solid additives were then mixedand the mixture was allowed to stir for 10 minutes. Two separatepipelines were used for pumping the oil phase and water. The two linesjointed together to form one single line and hence the beginning of thecreation of the emulsion. Four different configurations were used inpreparing the gel.

valves lined up to avoid the emulsion passing through the static mixerand lines not equipped with any restriction;

valves lined up to pass through static mixer and the choke, whichcreated a pressure drop equal to 250 psi before reaching the outlet.

emulsion not passing through the static mixer and passing through thechoke.

emulsion passing through the static mixer without the choke installed.

As the oil phase was mixed in the tub, samples were collected and usinga blade-type mixer, water was mixed in to verify the homogeneity of theemulsion. The tests reproduce previous results achieved in thelaboratory. Due to the small volume, the temperature of the tubincreased. To ensure dissolution of the polymer, for the weightedsolution, the polymer was gently added immediately after the addition ofthe emulsifier. The mixture was allowed to stir for no less than 10minutes with the other chemicals being added as was previously done.

The oil phase was weighted successfully with the addition of barite.After the addition of water through the static mixer, a homogenousemulsion was formed. The oil phase was measured using a pressurized mudbalance and the density was 1.69 g/cm³.

Overall, the results were as expected. The addition of the choke didprovide the pressure drop needed to flip the emulsion from its oil phaseto the water phase and produce a strong gel with a pressure drop of 250psi. In absence of a choke, the static mixer alone was not enough energyproduced to flip the emulsion.

What is claimed is:
 1. A plugging fluid for plugging a subterraneanformation zone surrounding a drill hole essentially consisting of anemulsion comprising a dispersed aqueous phase containing an aqueous baseand a continuous hydrophobic phase containing a polymer consisting of agrafted cellulose ether derivative, a surfactant and a crosslinkingactivator of the polymer.
 2. The plugging fluid of claim 1, wherein thepolymer is a grafted hydroxyalkylcellulose polymer derivative graftpolymer.
 3. The plugging fluid of claim 2, wherein the polymer is2-hydroxyethyl cellulose grafted with vinyl phosphonic acid.
 4. Theplugging fluid of claim 3, wherein the ratio of 2-hydroxyethyl celluloseto vinyl phosphonic acid monomers in the graft polymer ranges from about5% to about 20%.
 5. The plugging fluid of claim 1, wherein thecrosslinking activator of the polymer are selected from the groupsconsisting of compounds providing iron III, boron, titanium IV,zirconium IV, aluminum III and antimony V ions; compounds containingdivalent calcium and magnesium ions, amines, acetates, sodium hydroxide,potassium hydroxide, sodium carbonate, potassium carbonate, sodiumacetate, ammomia, ammonia generating compounds, chelating agents, sodiumgluconate, sorbitol, and glycerol and mixture thereof.
 6. The pluggingfluid of claim 1, wherein the oil is present in a concentration of fromabout 5 to about 70 wt %, and the water is present in a concentration offrom about 30 to about 95 w %.
 7. A plugging fluid comprising from about10 to 25 wt % of diesel oil, 0.1 to 3 w % of a polyamide derivative, 0.1to 5 w % of a 2-hydroxyethylcellulose grafted with vinyl phosphonicacid, 70 to 90 wt % of fresh water, 0.5 to 1.5 w % of magnesium oxide,0.5 to 1 w % of sodium carbonate, 0.1 to 1 w % of sodium tetraboratepentahydrate and 0.1 to 1 w % of thehydroxyethylethylene-diaminetracetic acid.
 8. A plugging fluid forplugging a subterranean formation zone surrounding a drill holeessentially consisting of an emulsion comprising a dispersed aqueousphase containing an aqueous base and a continuous hydrophobic phasecontaining a polymer consisting of a grafted cellulose ether derivative,a surfactant and a crosslinking activator of the polymer, wherein saidcrosslinking activator comprises an admixture consisting of magnesiumoxide, sodium carbonate, hydroxyethylethylene-diaminetriacetic acid, andsodium tetraborate.